Public speaking is an “advanced” soft skill: the ability to speak well in front of audiences large and small can take years of training and practice to truly master. But the 80/20 rule works here too, so I've listed my tips for the first 20% that can make anyone an effective public speaker.
#1 Public speaking is a people thing.
Only heads of State and PR pukes deliver meticulously-practiced, pre-written speeches word-for-word. And there's a reason: it's tedious to listen to such speeches. So unless you are delivering new information where the slightest mistake could cost you your job or your life, don't orate.
People enjoy watching other people -- it's human nature. One of the most effective ways to engage your audience is to be yourself! Act in a way that is natural for you, and give folks a glimpse into your personality. Your audience will feel like they are getting to know you, in addition to learning about your topic.
Being yourself in front of an audience, of course, requires an excellent command of the proto-soft skill: acting with confidence.
#2 Make good use of your Communications 101 skills. Know your material, know your audience, give your audience a reason to care about your topic, and organize your thoughts.
#3 Don't read your slides. Reading your slides word-for-word can be insulting to the audience since it implies they can't read. Oops. It also broadcasts that you don't have any additional knowledge or perspective to add on your topic. In other words, you are wasting their time. Reading the slides is a guaranteed way to kill your speech dead.
There are two techniques you should apply to avoid the pitfall. First, know your topic (d'oh). If you don't know the topic, cram before the speech or get someone else to give it.
Second, distill your slides down to just a few words: a good goal is no more than 5 bullet points per slide, and no more than 5 words per bullet point. If you can, try not to let a bullet point exceed one line! This lets the audience get a gist of what you are going to talk about, but makes them listen to you for the information. Which is cool, since that's the point of attending a speech. ;-)
#4 Speak loudly. This is a sort of “stupid pet trick” for public speaking, but it really works. Don't shout, but try to speak loudly. Not only with that ensure folks can hear you, but it's a great trick to convey energy and maybe enthusiasm -- which in turn will help your audience engage.
#5 Speak slowly when you need to compensate for being hard to understand. Do you naturally speak very fast? Do you have a strong accent in the language you using for the speech? Folks who don't know you might need a little help adjusting to your speaking style. Speaking loudly will definitely help, but you can also compensate for an accent or a rapid-fire delivery by simply speaking a little more slowly.
#6 Rehearse your speech in advance to time it. It certainly won't hurt to practice the material, but the point of this technique is to tune your material to fit into the time allotted for your speech. Running long is frustrating to audiences, as is running short (although this is a fairly rare problem).
Powerpoint has an awesome feature to automatically time your run-through: Slide Show > Rehease Timings. If you find you are running too long, condense your material! The delivery time of a speech almost always grows 10%-20% in front of a live audience.